The invention concerns a scattered light range of sight measurement instrument which measures the amount of light scattered by a light beam as it traverses a gas sample from which the density of particulates in the gas sample can be established. With this information, the sight range or viewable distance through the sample can be determined.
Stray or scattered light measuring instruments are used for determining the presence of particles (aerosols, dust, etc.) in venting conduits as well as in the atmosphere as a whole. For example, the carbon particle emissions from a chimney can be determined in this way. Scattered light measuring instruments are further used in street traffic, ocean shipping and air transport, and they are used, when appropriate, to generate a warning signal when a predetermined range (or distance) of sight becomes too low, e.g. exceeds a lower threshold value.
Such instruments employ either the transmission principle or the light scattering principle.
Range of sight measuring instruments employing the transmission principle, also sometimes referred to as transmissometers, direct a predefined amount of light into one end of a measuring distance. The emitted light is received by a light receiver located at the other end of the measurement distance where the incoming light volume is measured. If there are no particles along the measuring distance, the receiver receives 100% of the emitted light. The mathematical relationship between the length of the measuring distance and the transmission value measured at the receiver can be used to determine the distance or range of sight through the air or gas sample being measured.
Instruments which, for example, include an optical reflector and direct the light beam several times over the measurement distance, and only then determine the amount of received light, are also transmissometers.
Sight range measuring instruments which measure light scattered by particulates in the sample being measured also employ a light emitter and a light receiver. The emitter and receiver are arranged so that an emitted light beam from the light emitter and a received light beam striking the light receiver cross each other to prevent light from the emitter from directly reaching the light receiver.
When the light emitter is on one side (or upstream) of the measurement zone and the light receiver is on the other side or downstream of the zone, and particulates in the gas sample are to be detected, the so-called forward-scattering principle is employed.
On the other hand, if the light emitter and receiver are both on the same side of the measurement zone, the so-called back-scatter principle is employed.
Since the two light beams cross each other, no light from the emitter can directly reach the light receiver. The receiver can only receive a portion of the emitted light beam when the emitted light strikes a particle inside the measurement zone, because the particle will scatter the light in multiple directions. Accordingly, as the density of the particles in the measurement zone increases, the light receiver receives proportionally more light, which indicates a reduced viewing range or distance through the sample.
Although scattering light instruments have many advantages as compared to transmissions instruments, a significant disadvantage of them is that with good sight only a very small amount of light is received by the light receiver. For example, the testing of the functionality of the individual components of the instrument and assembly groups requires additional steps. If the light emitter were to cease to operate due to a defect, there would be no light that is directed or scattered towards the light receiver even when the particle concentration in the measurement zone is high. Instead, the instrument would interpret such a reading as indicating an extremely good or long range of sight. According to published German patent application No. DE 19 05 016, such errors are precluded by closing the light path from the light scattering particles to the light receiver at regular, short intervals, during which no scattered light from the particles can reach the light receiver. During this time interval, a predetermined amount of test light is directed from the light emitter directly to the light receiver. In this manner, several components of the light scattering instrument can be monitored with regard to their functionality. However, a disadvantage of this is that this manner of checking for defects fails to recognize a fairly large number of possible defects. For example, it cannot be determined when the emitted light beam and/or the received light beam are completely interrupted by a large surface obstacle which would prevent light from reaching the light receiver. As a consequence, the scattered light instrument interprets this occurrence as indicating a large viewing distance. Further, any progressively increasing contamination of optical boundary surfaces cannot be recognized because the boundary surfaces are also not in the optical path of the test light directed to the light receiver.
Further suggestions for correcting typical problems encountered with such scattered light instruments come from German published patent application No. DE 44 01 755 A1. It proposes to integrate a transmissions arrangement in a forward-scattering instrument and by providing at least one additional light receiver, or an additional light receiver and an additional light emitter. This combined and switchable scattered light and transmitted light measurement is said to enable the correct detection of obstacles within the optical measuring distance, or to compensate for deposits on the optical boundary surfaces. The arrangement of DE 44 01 755 A1 however has the disadvantage that the use of an additional light receiver involves additional costs. Moreover, with such a transmissions measurement, effectively only half the length of the scattered light measurement distance is being tested. For this reason, the German publication points out that the additional light receiver should also have an additional light emitter. Aside from the resulting higher costs, a major disadvantage of such an arrangement is that the optoelectronic components frequently react differently to changed conditions. It can therefore not be assured that over a longer period of time this will lead to correct compensations for measuring errors due to contaminants on the optical boundary surfaces. The reason for this is that whenever small particle concentrations are present (and the line of sight is large), the useable amount of light is necessarily very small so that even relatively minor incorrect compensation values can lead to large measurement errors.